President Trump has been trying to undermine the special counsel investigation from the very beginning, but yesterday, he took it to a whole new level. He tweeted that the U.S. Attorney General Jeff Sessions should put a stop to the investigation, quote, "right now." White House spokeswoman Sarah Sanders later explained it this way.

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SARAH SANDERS: It's not an order. It's the president's opinion.

MARTIN: An opinion that couldn't materialize anyway because Jeff Sessions has recused himself from the Mueller investigation. Over and over, the president calls the probe a witch hunt, even though Mueller's team has secured indictments or guilty pleas from 32 people and several Russian companies. NPR's lead political editor Domenico Montanaro is here to get us up to speed on all this. Hey, Domenico.

DOMENICO MONTANARO, BYLINE: Hey there, Rachel.

MARTIN: So it's interesting to note how the president's lawyers have changed how they've been talking about the primary allegation here - whether or not the Trump campaign colluded with Russia in the 2016 election. First, it was a drumbeat of there was no collusion, there was no collusion. And, just recently, Rudy Giuliani's changed tactically and has said, hey, collusion's not a crime.

MONTANARO: Well, where's Rudy Giuliani today? (Laughter) You know?

MARTIN: Right.

MONTANARO: He is kind of all over the place on a lot of this stuff. And that's because it's very intentional. I mean, it's a public relations battle that Giuliani is waging as the president's attorney outside the White House dealing with this investigation. And there's a big reason for that. That's because so much of what happens here is going to depend on whether or not Congress, after these elections and when they finally get a report from Mueller, if they do get a report from Mueller, if Congress will decide to try to impeach the president.

That's because the Justice Department guidelines are pretty clear that the Justice Department frowns upon going after a sitting president. They don't believe that you can because of the constitutional separation of powers. So that's going to depend a lot on whether or not, you know, for example, the president, Mueller believes, obstructed justice when it came to firing James Comey or trying to shut down the Russia investigation, which we saw the White House tapdancing on yesterday as to whether or not the president was directing the attorney general to end the investigation.

MARTIN: Right. And we should point out collusion itself is not articulated as a crime, but conspiracy is.

MONTANARO: Right. I mean, we have been pointing that out for months - right? - that collusion is not really the issue. It's about - colluders, as I've said, have to worry about conspiracy. And conspiracy against the United States is most certainly a crime, and one, by the way, that Mueller has already used against a number of suspects in this ongoing Russia probe.

MARTIN: And it is interesting to think about, does conspiracy have to happen in private, or can it be out in the open? Does collusion happen in private, or can it be out in the open? And people looked at the president's tweet yesterday calling on Jeff Sessions to shut down the investigation and pointed at that and said, you know, this pretty clearly looks like obstruction of justice out in the open.

MONTANARO: You know, and the thing is, I wouldn't look at the tweets and whether or not Mueller's team is looking at the tweets - and, of course, they are - as something to say that, oh, they're going to hang their hat and the entire investigation on tweets. You know, investigators will make a timeline of all of these events, big pieces of things that happened - when was Comey fired, what dates Comey wrote these contemporaneous notes. And then, how does that coincide with the tweets, the president's state of mind? And that's so much of why Mueller and his team want to also talk to the president to kind of fill in some of these blanks.

MARTIN: Right. So key to the special counsel's investigation, though, is really this possible interview. Robert Mueller wants to sit down face-to-face with President Trump and ask him some questions. Where does that effort stand right now?

MONTANARO: Well, Rudy Giuliani yesterday said that he thinks it could be unlikely. But that could also just be some gamesmanship by Giuliani because, clearly, the president's team would like to curtail whatever information is given to Mueller's team. You know, they'd much rather do something in writing, for example. There were reports that Mueller's team had decided that, well, maybe we'll curtail the number of obstruction questions. So this is all up in the air. It's clearly the next phase and a really important phase in the next part of this investigation.